1. Field of the Invention
The present invention relates to a blood pressure monitor including an inflatable cuff.
2. Related Art Statement
There is known a blood pressure (BP) monitor which includes an inflatable cuff adapted to be wound around a body portion, e.g., upper arm, of a living subject, e.g., patient, to press the body portion. The BP monitor functions as an automatic BP measuring device which periodically measures a BP value of the subject by increasing the cuff pressure and thereby pressing the body portion of the subject. However, if the period or interval of the BP measurements effected by the BP monitor is shortened for improving the accuracy of monitoring of subject's blood pressure, the frequency of pressing of subject's body portion is increased, which causes the subject to feel discomfort.
In the above-indicated background, it has been proposed to increase the pressure of an inflatable cuff being wound around a body portion of a living subject, up to a predetermined value, detect a pulse wave that is a pressure oscillation produced in the cuff, and estimate a BP value of the subject based on the magnitude of the pulse wave. This technique is disclosed in, e.g., Japanese Patent Application laid open for inspection purposes under Publication No. 61(1986)-103432, or Japanese Patent Application laid open for inspection purposes under Publication No. 60(1985)-241422.
Regarding the above-indicated conventional BP monitor techniques, however, there are known some cases where it is difficult to detect a change of magnitudes of pulse waves which reflects a change of blood pressure of a living subject, if BP values are estimated based on the pulse waves detected at a considerably low cuff pressure, which contributes to reducing the discomfort felt by the subject. More specifically described, respective amplitudes of pulses of a pulse wave which is detected from an inflatable cuff being wound around a body portion of a living subject whose blood pressure is normal, has an envelope indicated at solid line in the graph of FIG. 6. In contrast, amplitudes of pulses of a pulse wave obtained from a living subject whose blood pressure is low, has an envelope indicated at one-dot chain line in FIG. 6. In the case where amplitudes of pulses of a pulse wave are detected at a considerably low cuff pressure, e.g., pressure, P.sub.K, in FIG. 6, an amount of change of the pulse amplitudes with respect to an amount of change of blood pressure of a living subject may be too small. Thus, when the BP monitor is used at the low cuff pressure P.sub.K, it may not be able to monitor the blood pressure of the subject with high accuracy.
There is also known a continuous BP monitor which includes an inflatable cuff which is adapted to be wound around a body portion of a living subject to press the body portion; a blood pressure measuring device which measures a blood pressure of the subject by changing a pressure in the cuff; a pressure pulse wave sensor which is adapted to be pressed against a distal section of the artery located on a distal side of the cuff wound around the body portion, so as to detect a pressure pulse wave which is produced from the distal section of the artery; a relationship determining means which determines a relationship between blood pressure and magnitude of pressure pulse wave, based on the blood pressure measured by the blood pressure measuring device and a magnitude of the pressure pulse wave detected by the pressure pulse wave sensor; a blood pressure determining means which successively determines a blood pressure of the subject according to the determined relationship based on a magnitude of each of successive heartbeat-synchronous pulses of the pressure pulse wave detected by the pressure pulse wave sensor; and a display which displays the blood pressure values determined by the blood pressure determining means. This BP monitor is disclosed in, e.g., Japanese Patent Application laid open for inspection purposes under Publication No. 1(1989)-214338 or Japanese Utility Model Application laid open for inspection purposes under Publication No. 2(1990)-82309.
In the prior continuous BP monitor, the condition under which the pressure pulse wave sensor is pressed against subject's artery may be changed due to, e.g., a physical motion of the subject. Hence, in order to improve the accuracy of BP values determined by the BP determining means, the relationship between blood pressure and magnitude of pressure pulse wave is updated at a predetermined period. However, the updating of the relationship needs a blood pressure measurement of the blood pressure measuring device including the inflation of the cuff. In addition, since the pressure pulse wave sensor is set on the distal side of the cuff, the continuous BP determination of the BP determining means is interrupted by the inflation of the cuff. This problem is exaggerated if the period of updating of the relationship is shortened for improving the accuracy of the continuous BP monitoring.